<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .box1 {
      width:50%;
      height:300px;
      background: #ff6670;
    }
    .box2{
      width: 200px;
      height: 200px;
      background: #ff7800;
    }
    .box3{
      width: 100%;
      height: 100%;
      background: #bb007d;
    }
    body{
      margin: 0;
      height: 100%;
    }
    html{
      /* margin: 0; */
      height: 100%;
    }
  </style>
</head>
<body>
  <div class="box1">
   这是一段文字
  </div>
  <div class="box2">
    这是第二段文字
  </div>
  <div class="box3">
    这是第三段文字
  </div>
</body>
</html>